In the art of facsimile recording a web such as electrolytic paper is fed between an elongate linear marking electrode and a scanning electrode which moves repeatedly across the paper transversely of the direction in which the paper is fed. The scanning electrode moves in a path along a recording zone defined by its own path and the edge of the linear electrode which is parallel to the path of the scanning electrode. As the scanning electrode traverses the paper recording web electrical signals applied between the two electrodes mark graphic information on the web, line by line, similarly as television signals are traced on a cathode ray screen.
One form of scanning electrode is a conductive signal marking stylus carried on a belt which conveys the stylus on two or more styli across the web along the recording zone. Examples of such belt-supported styli are found in U.S. Pat. Nos. 2,879,129 to M. Alden, 3,363,261 to K. Maierskofer and 3,369,250 to T. H. Gifft. The scanning belt typically has feed perforations or tooth elements interengaging with a driven pulley, roll or spool, which drives the belt in an orbital path and repeatedly conveys the stylus or styli along the scanning zone across the paper as electrical signals representing successive lines of a frame of graphic information are applied between the stylus and linear electrode during consecutive line scans by the stylus.
One requirement of such belt-supported stylus scanning is that the stylus or styli must start each line scan at the same position transversely of the web as each line of electrical signals is applied. If the transverse starting position of the stylus were to change from line to line in a frame some lines would be offset from others thereby distorting the graphic information in the frame or rendering it illegible. This problem of precisely repeating the stylus starting position at the beginning of consecutive line scans is particularly difficult when two or more styli are mounted on the belt as is usually desirable. For example, with two styli on the belt the spacing between the styli must be the same measured forwardly (or rearwardly) from each stylus in order to insure that a constant speed drive pulley will present each stylus at the same starting position transversely of the recording web at each successive line scan. The correct spacing is not simply a matter of marking measured locations on a belt and positioning the styli at the measured locations. The stylus should be located within a few thousandths of an inch of exact position, an accuracy not consistently achievable by eye. Moreover the stylus must be precisely located with respect to the feed perforations, teeth or other feed elements on the belt.
Accordingly the object of the present invention is to provide a stylus scanning belt whereon the stylus is located with improved accuracy.